In modern aircrafts the wing profile is crucial for the flight. If this profile is altered for some reason, the flying characteristics and consequently the stalling characteristics of the wing are significantly deteriorated.
The wing profile hardly changes in normal conditions, except when ice is formed on the wing surface for various reasons. In fact, ice formation on the wings of aircrafts has become a noticeable hazard in air traffic, since it has been noted that the ice layers may grow a thickness of up to one inch, whereby the flying characteristics of the wing are substantially weakened. Ice formation on the wing has proved to be the very reason for a number of recent passenger flight accidents. Ice formation involves a second drawback: in many jet planes the motors and their air intakes are located at the rear end of the fuselage, implying that, as the wing is bent at takeoff, ice is detached and absorbed directly into the air intakes of the motor, causing the turbine wings of the motor to break.
Ice formation may occur in various ways: during the flight weather conditions may be such that ice starts forming on the wings; also weather conditions during taxiing may generate ice on the surface of the wing; however, the most unexpected situation arises, when an aircraft having flown at high altitudes at a low temperature (e.g. -50.degree. C.) accumulates a thick ice layer on its wings after landing at the airport. This is due to the fact that the fuel has been deeply cooled in its tank during the flight. The design of fuel tanks allow the fuel to get into contact with the upper surface of the wing, the upper surface of the wing being extremely cooled and accumulating ice on the surface, although the air temperature at the airport would be above zero. This ice formation process has been difficult to verify and has caused surprises in air traffic. Aircraft manufacturers have of course taken measures to eliminate the risks caused by ice formation. The most common method is spraying the wings with glycol liquid, which melts snow and ice that may have accumulated on the wing. Another method consists in checking the humidity and the temperature of outside air, enabling to anticipate conditions in which ice formation occurs, and to conduct combustion heat from the motors to the front edge of the wing to melt the ice. A special ice detector is further provided at the front end of the fuselage, an ice formation alarm going off if ice is formed on the surface of the detector. In this situation, precautions can be taken to prevent ice formation. Nevertheless, this detector does not indicate whether ice has accumulated precisely on the surface of the wing.
Finnair in particular has implemented a very simple ice detection in their aircraft: a special kind of strips are fixed on the surface of the wing, the strips fluttering in the air current if the wing surface is bare and no ice is present. Furthermore, Finnair has used a kind of clearly visible flange that can be observed from the ground. If the flange and the scale attached to it cannot be clearly seen, this implies that there is ice on the wing surface.
As to prior art, we note that the most common solution is a detector attached to the front end of the fuselage, the detector consisting of a short, vibrating stick which vibrates continuously at its natural resonance frequency. If ice is formed on the vibrator stick, the ice and its high viscosity will change the vibration frequency and attenuate the vibration amplitude. A slightly different detector based on ultrasonics placed at any point on the wing, has been recently launched. This detector consists of a metal disc mounted on the surface of the wing plane, and an ultrasonic crystal below the disc causes the metal disc to vibrate. If water is present on the disc, the viscosity of water is low enough not to attenuate the vibration. Conversely, if the water on the disc freezes, the viscosity is altered abruptly, and the vibration amplitude decreases noticeably, the decrease being detectable at the ultrasonic crystal under the disc. This detector has the advantage of being mountable at the point of the fuel tank, whereby it detects the ice formation at the very point where this would be hazardous. The drawback of this detector is difficult mounting, since it must be mounted inside the fuel tank, which is a dangerous and expensive solution because of explosion risks, among others. In addition, this detector detects ice formation only at a given, spot-like point, which is not sufficient for total security.
FI patent specification 61249 describes an ultrasonic detector enabling to detect the presence of "black ice" on a bituminous road by transmitting an ultrasonic pulse along a thin thread, the pulse being subsequently reflected at the end of the thread. If the thread is covered with ice, no echo pulse will be obtained at the end of the thread. This detector has the inconvenience that partial freezing of the thread or the thread holders pressing the thread to the support surface may cause an extra reflection or false echo which reduces the operating reliablity of the detector. Moreover, the detection electronics required for the detector is relatively complicated. It is known that aviation electronics must be as reliable and straightforward as possible.